Measurement of the lung volumes is important clinically because many pathologic states can alter specific lung volumes or their relationships to one another. The lung volumes, however, can also change for normal physiologic reasons. Changing from a standing to a supine posture decreases the FRC because gravity is no longer pulling the abdominal contents away from the diaphragm. This decreases the outward elastic recoil of the chest wall, as noted in Chapter 2, Figure 2–14. The RV and TLC do not change significantly when a person changes from standing to the supine position. If the FRC is decreased, then the ERV will also decrease (Figure 3–2), and the IRV will increase. The VC, RV, and TLC may decrease slightly because some of the venous blood that collects in the lower extremities and the abdomen when a person is standing returns to the thoracic cavity when that person lies down.
Determination of the lung volumes can be useful diagnostically in differentiating between 2 major types of pulmonary disorders—the restrictive diseases and the obstructive diseases. Restrictive diseases like alveolar fibrosis, which reduce the compliance of the lungs, lead to compressed lung volumes (Figure 3–3). The increased elastic recoil of the lungs leads to a lower FRC, a lower TLC, a lower VC, and lower IRV and ERV, and it may even decrease the RV. The VT may also be decreased, with a corresponding increase in breathing frequency, to minimize the work of breathing.
Obstructive diseases such as emphysema and chronic bronchitis cause increased resistance to airflow. Airways may become completely obstructed because of mucus plugs as well as because of the high intrapleural pressures generated to overcome the elevated airways resistance during a forced expiration. This is especially a problem in emphysema, in which destruction of alveolar septa leads to decreased elastic recoil of the alveoli and less radial traction, which normally help hold small airways open. For these reasons, the RV, the FRC, and the TLC may be greatly increased in obstructive diseases, as seen in Figure 3–3. The VC and ERV are usually decreased. The breathing frequency may be decreased to reduce the work expended overcoming the airways resistance, with a corresponding increase in the VT.
The spirometer is a simple device for measuring gas volumes. The traditionally used water sealed spirometer, shown in Figure 3–4, consists of an inverted canister, or “bell,” floating in a water-filled space between 2 concentrically arranged cylinders. The space inside the inner drum, which is closed off from the atmosphere by the bell, is connected to tubing that extends to a mouthpiece into which the person breathes. As the person breathes in and out, gas enters and leaves the spirometer, and the bell then floats higher (during expiration) and lower (during inspiration). The top of the bell is connected by a pulley to a pen that writes on a rotating drum, thus tracing the person’s breathing pattern.
Determination of lung volumes and capacities with a spirometer. A: Schematic representation of a water-sealed spirometer. B: Determination of the tidal volume, vital capacity, inspiratory capacity, inspiratory reserve volume, and expiratory reserve volume from a spirometer trace.
As is evident from Figure 3–4, the spirometer can measure only the lung volumes that the subject can exchange with it. As is the case with many pulmonary function tests, the subject must be conscious and cooperative and understand the instructions for performing the test. The VT, IRV, ERV, IC, and VC can all be measured with a spirometer (as can the forced expiratory volume in 1 second [FEV1], forced vital capacity [FVC], and forced expiratory flow [FEF25%–75%], as discussed in Chapter 2). The RV, the FRC, and the TLC, however, cannot be determined with a spirometer because the subject cannot exhale all the gas in the lungs. The gas in a spirometer is at Ambient Temperature, Pressure, and water vapor Saturation (ATPS), and the volumes of gas collected in a spirometer must be converted to equivalent volumes in the body. Other kinds of spirometers include rolling seal and bellows spirometers. These spirometers are not water-filled and are more portable.
Measurement of Lung Volumes Not Measurable with Spirometry
The lung volumes not measurable with spirometry can be determined by the nitrogen-washout technique, by the helium-dilution technique, and by body plethysmography. The FRC is usually determined, and RV (which is equal to FRC minus ERV) and the TLC (which is equal to VC plus RV) are then calculated from volumes obtained by spirometry.
In the nitrogen-washout technique, the person breathes 100% oxygen through a 1-way valve so that all the expired gas is collected. The concentration of nitrogen in the expired air is monitored with a nitrogen analyzer until it reaches zero. At this point, all the nitrogen is theoretically washed out of the person’s lungs. The total volume of all the gas the person expired is determined, and this amount is multiplied by the percentage of nitrogen in the mixed expired air, which can be determined with the nitrogen analyzer. The total volume of nitrogen in the person’s lungs at the beginning of the test can thus be determined. Nitrogen constitutes about 80% of the person’s initial lung volume, and so multiplying the initial nitrogen volume by 1.25 gives the person’s initial lung volume. If the test is begun at the end of a normal tidal expiration, the volume determined is the FRC:
Total volume expired × % N2 = original volume of N2 in lungs
Original volume of N2 in lungs × 1.25 = original lung volume
The helium-dilution technique, like other indicator dilution techniques, makes use of the following relationship: If the total amount of a substance dissolved in a volume is known and its concentration can be measured, the volume in which it is dissolved can be determined. For example, if a known amount of a solute is dissolved in an unknown volume of solvent, and the concentration of the solute can be determined, then the volume of solvent can be calculated:
Amount of solute (mg) = concentration of solute (mg/mL) × volume of solvent (mL)
In the helium-dilution technique, helium is dissolved in the gas in the lungs and its concentration is determined with a helium meter, allowing calculation of the lung volume. Helium is used for this test because it is not taken up by the pulmonary capillary blood and because it does not diffuse out of the blood, and so the total amount of helium does not change during the test. The person breathes in and out of a spirometer filled with a mixture of helium and oxygen, as shown in Figure 3–5. The helium concentration is monitored continuously with a helium meter until its concentration in the inspired air equals its concentration in the person’s expired air. At this point, the concentration of helium is the same in the person’s lungs as it is in the spirometer, and the test is stopped at the end of a normal tidal expiration, in other words, at the FRC.
The helium-dilution technique for the determination of the functional residual capacity. A: Before the test, the spirometer is filled with a mixture of helium (denoted by the dots) and oxygen. The concentration of helium is determined by the helium meter. B: The subject breathes from the spirometer until the helium concentration in the lungs equilibrates with that in the spirometer. During the equilibration period, the subject’s expired carbon dioxide is absorbed and oxygen is added to the spirometer at the subject’s oxygen consumption rate. The helium concentration and spirometer volume are determined after equilibration, when the subject is at functional residual capacity.
The FRC can then be determined by the following formula (total amount of He before test = total amount of He at end of test):
FHEi Vspi = FHEf (Vspf + VLf)
That is, the total amount of helium in the system initially is equal to its initial fractional concentration (Fhei) times the initial volume of the spirometer (Vspi). This must be equal to the total amount of helium in the lungs and the spirometer at the end of the test, which is equal to the final (lower) fractional concentration of helium (Fhef) times the final volume of the spirometer (Vspf) and the volume of the lungs at the end of the test (Vlf). Since it may take several minutes for the helium concentration to equilibrate between the lungs and the spirometer, in practice, CO2 is absorbed from the system and oxygen is added to the spirometer at the rate at which it is consumed by the person. A correction factor is used to account for the small amount of helium that does dissolve in the blood during the test. Both the nitrogen-washout and helium-dilution methods can be used on unconscious patients.
A problem common to both the nitrogen-washout technique and the helium-dilution technique is that neither can measure trapped gas: the nitrogen trapped in alveoli supplied by closed airways cannot be washed out and the helium cannot enter alveoli supplied by closed airways. Furthermore, if the patient’s lungs have many alveoli served by airways with high resistance to airflow (the “slow alveoli” discussed at the end of Chapter 2), it may take a very long time for all the nitrogen to wash out of the patient’s lungs or for the inspired and expired helium concentrations to equilibrate. In such patients, measurements of the lung volumes with a body plethysmograph are much more accurate because they do include trapped gas.
The body plethysmograph makes use of Boyle’s law, which states that for a closed container at a constant temperature, the pressure times the volume is constant. The body plethysmograph, an expensive piece of equipment, is shown schematically in Figure 3–6.
The use of the body plethysmograph for the determination of the functional residual capacity. The subject is seated in the small airtight chamber and breathes through the apparatus shown. By monitoring the subject’s airflow with a pneumotachograph, the operator can briefly occlude the subject’s airway at end expiration. As the subject makes an inspiratory effort against the closed airway, the pressure in the chamber (Pbox) increases and the pressure at the subject’s mouth (Pm) decreases. The subject’s functional residual capacity can then be calculated.
As can be seen from the figure, the body plethysmograph is an airtight chamber large enough so that the patient can sit inside it. The patient sits in the closed plethysmograph, or “box,” and breathes through a mouthpiece and tubing. The tubing contains a sidearm connected to a pressure transducer (“mouth pressure”), an electrically controlled shutter that can occlude the airway when activated by the person conducting the test, and a pneumotachograph to measure airflow, allowing the operator to follow the subject’s breathing pattern. A second pressure transducer, which must be very sensitive, monitors the pressure in the plethysmograph (“box pressure”).
After the subject breathes through the open tube for a while to establish a normal breathing pattern, the operator closes the shutter in the airway at the end of a normal tidal expiration. At this point, the subject breathes in for an instant against a closed airway. As the subject breathes in against the closed airway, the chest continues to expand and the pressure measured by the transducer in the plethysmograph (Pbox) increases because the volume of air in the plethysmograph (Vbox) decreases by the amount the patient’s chest volume increased (ΔV):
Pboxi × Vboxi = Pboxf × (Vboxi − ΔV) (1)
where (Vboxi – ΔV) = Vboxf
That is, the product of the initial box pressure times the initial box volume must equal the final box pressure times the final box volume (the initial box volume minus a change in volume), according to Boyle’s law. Of course, direct measurement of box volume, which is really equal to the volume of the plethysmograph minus the volume occupied by the patient, is impossible, and so the plethysmograph is calibrated by injecting known volumes of air into the plethysmograph and determining the increase in pressure. After such a graph of pressure changes with known changes in volume has been constructed, the ΔV in Equation (1) can be determined.
The product of the pressure measured at the mouth (Pm) times the volume of the patient’s lungs (Vl) must also be constant during the inspiration against a closed airway. As the patient breathes in, the volume of the lungs increases by the same amount as the decrease in the volume of the box determined in Equation (1) above (ΔV). As the lung volume increases, the pressure measured at the mouth decreases, as predicted by Boyle’s law:
PMi × VLi = PMf × (VLi + ΔV) (2)
The ΔV in Equation (2) is equal to that solved for in Equation (1) and Vli is now solved for. It is the FRC, since the airway was occluded at the end of a normal tidal expiration. In current practice, the patient makes several panting inspiratory efforts against the closed airway, and all the calculations described above are made automatically by a computer receiving inputs from the pressure transducers.